Terminating resistor? Some users may not be familiar with this term, so let's introduce it through a question from a customer. Take our UN120 economy PLC as an example. It has two communication ports, but one of them is a terminal block type, as shown below:
Customers can easily understand the A, B, and shielded ground terminals when they see this port description, but what is the purpose of the R terminating resistor, in what situations should this resistor be connected, and what is its resistance value?
Before revealing the answer, let's explain the terminating resistor in detail!
What is a terminating resistor?
A resistor connected in parallel on a pair of communication lines at both ends of a linear network (the two communication ports furthest apart).
The terminating resistance value depends on the impedance characteristics of the cable and is independent of the cable length. RS-485/RS-422 generally uses twisted-pair cable (shielded or unshielded) for connection, and the terminating resistance is generally between 100 and 140 Ω, with a typical value of 120 Ω. In actual configuration, a terminating resistor is connected to each of the two terminal nodes of the cable, i.e., the nearest end and the farthest end. Nodes in the middle should not have terminating resistors connected, otherwise communication errors will occur.
So what is the purpose of adding a terminating resistor? Generally speaking, a terminating resistor is used to eliminate signal reflections in communication cables. According to transmission line theory, a terminating resistor can absorb reflected waves on the network, effectively enhancing signal strength, thus enabling long-distance communication.
There are two reasons for signal reflection:
(1) Impedance discontinuity: When a signal encounters a cable with very low or even no impedance at the end of the transmission line, the signal will be reflected at this point. The principle of this signal reflection is similar to the reflection of light when it enters another medium from one medium. The way to eliminate this reflection is to connect a terminating resistor of the same size as the characteristic impedance of the cable at the end of the cable to make the impedance of the cable continuous. Since the transmission of signals on the cable is bidirectional, a terminating resistor of the same size can be connected across the other end of the communication cable.
(2) Impedance mismatch: Another reason for signal reflection is the impedance mismatch between the data transceiver and the transmission cable. In high-frequency circuits, when the signal frequency is very high, the signal wavelength is very short. When the wavelength is comparable to the length of the transmission line, the reflected signal superimposed on the original signal will change the shape of the original signal, causing the signal waveform to be distorted (dipping or bulging). Reflection caused by this reason is mainly manifested in the chaotic data of the entire network when the communication line is in idle mode.
Based on the above understanding of the terminating resistor, we can explain the customer's problem. In other words, if the customer uses the UN 124XP terminal serial port for long-distance 485 communication, this resistor needs to be connected to ensure smooth long-distance communication. The resistance value is 120Ω.
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